Storing renewable energy is just as important as generating it, and flow batteries might be one of the most promising ways to do that. While there are plenty of hurdles to jump over in perfecting the tech, a team of Harvard engineers has been making strides over the past few years with organic flow batteries, and has now tested a new molecule that makes for the longest-lasting, high-performance organic flow battery so far.

Flow batteries are built with two liquid electrolytes that are stored in external tanks and piped into the cell as needed. During charging and discharging, they pass electrons back and forth through a membrane in the cell, and their storage capacity and power output can be tweaked by changing the size of the tanks and membrane, respectively.

Traditionally, the best results from this type of battery came from electrolytes of vanadium and bromine dissolved in acid, but these chemicals can be costly and caustic. Organic alternatives to vanadium were found in the form of quinones, molecules similar to those used in plants and animals to store energy.

In 2014, the Harvard team started experimenting with over 10,000 types of quinones, gradually figuring out which ones performed best. Later, they replaced bromine with ferrocyanide and switched the acid for an alkaline mixture, then singled out a modified version of vitamin B2 as a particularly useful quinone. And finally, last year they tweaked the recipe to run on neutral water.